They summoned the goddess Yogamaya and experienced her transfer the fetus within the womb of Devaki towards the womb of Rohini (Vasudeva's other wife who lived with her sister Yashoda through the river Yamuna, while in the village of cowherds at Gokulam). Hence the child conceived in a single womb https://cs.stackexchange.com/users/174464/southern-california-surrogacy